Localise the 'Welcome' article and other default content (such as category texts) #1179

I'm pretty sure we can leverage the recent installation changes and make the various default content installed at setup more localised. I can put the content onto Crowdin for translation to expedite this.

This is especially true of things like RTL languages and other non-Latin alphabets where we have an English article amongst the already localised content. Not a great advert for language support!